In 1973 there were 16 teams. In 1978 there were 18. In 1983 and 1988 there were 21. In 1993 there were 24 teams, 26 in 1998, and all later years in the grid have 30 teams.
I can't speak for the high number in the early 70s, but the later trend seems to indicate "more teams=more balanced scoring". One guess I might have is that 1973 did not use divisions, while the later years did.

But if you want a great look at how teams in the 80s were all basically the same aside from your top few players, compare the 1986 Oilers and the 1986 Leafs. Take off the top four scoring forwards and the top scoring defenseman from each team, and look at the roster. It's almost statistically identical; Toronto is actually offensively more powerful at that point.

That's not what the data shows. Scoring in the 1980s was more balanced than it has been from the mid 1990s to today.